This invention is concerned with conveyor belts, and in particular the invention relates to a spiral conveyor belt driven in part by a driving tower or cage, and to improvement in driving engagement between the driving cage and the upwardly spiraling belt.
In a related patent, U.S. Pat. No. 5,069,330, assigned to the assignee of the present invention, a spiral conveyor belt system was disclosed wherein an edge member or side plate includes one or more generally vertical grooves formed in its outer side. These vertical grooves were positioned to engage with protruding structure of cage bars of a spiral conveyor driving cage. The modular plastic conveyor belt had recessed rod heads on the connecting rods, and at least one of the generally vertical grooves was preferably positioned coincident with the rod head recess. The driving cage was disclosed as including ridges or protrusions, preferably formed in cage bar caps assembled onto the exterior of the cage bars. At any one time, at least some of the protrusions on the cage bars are engaged in grooves of the side plates, providing assistance in the driving of the spiral conveyor belt by the driving cage.
Other approaches have been suggested for providing more positive gripping engagement or "positive drive" between a driving cage and a spiral conveyor belt, particularly for high speed spiral conveyors. See Irwin U.S. Pat. No. 4,941,566, Roinestad U.S. Pat. No. 4,741,430 and Roinestad U.S. Pat. No. 4,852,720. The Irwin patent describes jackets or caps for the cage bars of a driving cage, which are rectangular in cross section. These caps provide grooves at the outer side of each driving cage bar. The grooves cooperate with steel connecting rod heads of a metal spiral conveyor belt. Although not every rod head becomes engaged in a cage bar groove, due to phase shifting in the spacings involved, some of the rod heads do become engaged. The rod head and groove arrangement is supposed to provide some driving assistance and establish less slippage of the spiral conveyor belt against the driving cage, which moves circumferentially faster than the belt in "overdriving" relationship.
The Roinestad patents disclose another type of "positive drive" for a spiral conveyor system. In the Roinestad patents, cage bar caps include linear vertical protrusions positioned to engage against protruding rod heads in a metal spiral conveyor belt. The vertical driving protrusions of the cage bar caps are square in cross-section and are intended to grip against the protruding belt rod heads and thus drive the belt, or a portion of the belt, at the same speed as the driving tower for a certain period or arc of movement. Since a spiral conveyor belt rises as it progresses, the rod heads in the Roinestad arrangement were to ride up on the cage bar protrusions until they were released at a vertical gap or interruption in the vertical protrusion of the cage bar. This would release the rod heads and allow the rod and belt edges to spring back, then subsequently engage a second, different cage bar protrusion farther back than the first. In this way, the Roinestad "positive drive" arrangement was intended to intermittently drive groups of protruding rod heads at the same speed as the driving tower. Thus, there would be a dwell time during which rod heads would be forced to travel with the bars of the driving tower, then the differential speed would cause a snapping or jerking disengagement, which was intended to occur at the vertical gap in the cage bar protrusion.
It is a principal purpose of the present invention to provide improved driving engagement between a spiral conveyor driving cage and a plastic conveyor belt, with structure which is relatively simple and advantageous over prior apparatus aimed toward the same purpose.